1. Field of the Invention
The present invention relates in general to an apparatus for controlling communications among computer systems wherein a plurality of arithmetic units, memory or storage units and input-output units are coupled to associated buses for mutual communication and in particular to an inter-bus coupling control apparatus for effecting the communications among the computer systems through the buses with a minimized number of required hard wares. More specifically, the present invention concerns a dead-lock control apparatus for excluding dead-locks which may possibly occur in the communications among the buses.
2. Description of the Prior Art
Lately, a so-called multi-computer system in which a plurality of computers are interconnected in a network is employed in an effort to enhance the reliability as well as the processing capability of the computer.
However, because the communications between the computers through the buses can not be effected more than one at a time, there arises a disadvantage in the system composed of more than two computers that the throughput of the overall system is reduced. To evade such disadvantage, it is conceivable that the system comprising a plurality of computers is divided into more than two subsystems which are adapted to operate independently of one another and are coupled to one another only when communications are desired among the different subsystems.
In the hitherto known systems, the communication between the buses, i.e. the inter-bus communication is performed with the aid of a computer by utilizing the buses to which a plurality of arithmetic units, storage units and the input-output units are connected. This system is certainly advantageous in that a flexiblity as well as a wide utility for general purposes can be attained in the control of the communication between the subsystems. However, it can not be evaded from such disadvantages that a large quantity of hard wares are required and, besides, a considerable time is necessary for the control of the communication due to the fact that the communications among the subsystems are realized through the computer.
On the other hand, in the prior known inter-bus coupler apparatus of a simplified configuration in which the hard wares required for controlling the performance of the inter-bus communication and the time for controlling the communication between the buses, are minimized, there is a drawback that a dead-lock or jamming may occur, when the requests for the inter-bus communication are simultaneously called for from both buses.